The surface of a silicon substrate is reformed by etching to predetermined flatness suitable for the process of manufacturing a semiconductor device. The etching method is generally classified to wet and dry processes. The dry process necessitates the energy beam irradiation, resulting in the easy formation of various faults such as electrostatic damages and distortions which would unfavourably affect on the properties of an obtained device. The wet process substantially put less damage on an obtained device, since the surface of a silicon substrate is treated by chemical reaction, although the substrate would be contaminated with impurities carried from an etching solution or the formation of oxide films on the surface.
The oxidation on the surface is one of the problems in the wet process. In order to prevent the oxidation, there has been proposed the method wherein a unbound end existent on the outmost surface of a silicon substrate is terminated with hydrogen to inhibit the spontaneous formation of oxide films. The contamination with impurities has been almost eliminated, as the purity of pure water to be used becomes higher. The process used by Radio Corporation of America undermentioned is the representative wet process for cleaning a silicon substrate.
PRECLEANING
Precleaning is an optional step. In this step, a silicon substrate is treated with oxidation by plasma, an organic solvent, a heated H.sub.2 O.sub.2 --H.sub.2 SO.sub.4 mixed solution or the like, to remove photoresist or other impurities from the surface of the substrate.
SC-1 CLEANING
The silicon substrate is then dipped in a H.sub.2 O--H.sub.2 O.sub.2 --H.sub.2 SO.sub.4 mixed solution in a quartz beaker, while the mixed solution is being stirred. After the silicon substrated is held a few minutes in contact with the mixed solution heated at 75.degree.-80.degree. C., the silicon substrate is cooled by continuously washing with ultrapure water. The treated silicon substrate is carried to a rinsing tank, and rinsed with ultrapure water therein.
REMOVAL OF NATURAL OXIDE FILM FROM SURFACE OF SUBSTRATE
The rinsed silicon substrate is immediately dipped in a H.sub.2 O--HF mixed solution. Herein, the removal of natural oxide films can be detected by observing the phenomenon that the mixed solution is repelled from the surface of the substrate. Thereafter, the silicon substrate is rinsed with ultrapure water, and then carried to the next SC-2 cleaning step without making the surface of the substrate dry.
SC-2 CLEANING
A H.sub.2 O--H.sub.2 O.sub.2 --HCl mixed solution received in a quartz beaker is heated at 75.degree.-80.degree. C. The silicon substrate is dipped about fifteen minutes in the heated solution. Therereafter, the silicon substrate is cooled by washing with ultrapure water, and then rinsed with ultrapure water.
DRYING
The silicon substrate is disposed in a holder and then set in a centrifugal dryer. The substrate is rotatingly rinsed in the centrifugal dryer using ultrapure water heated at 80.degree.-85.degree. C. The rinsed substrate is then dried with high-temperature isopropyl alcohol vapor.